Spring Data Rest - _links

Question

Edit 14/08/14 13:29

My next conclusion is that the hal+json format produced from my @RepositoryRestResource CrudRepository is incorrect.

The tutorial (http://spring.io/guides/gs/accessing-data-rest/) shows the output of a hypermedia Rest JPA entity as: (please note there is no "rel" element, and "links" is not an array)

{
   "_links" : {
       "people" : {
           "href" : "http://localhost:8080/people{?page,size,sort}"
       }
   }
 }

However, the reference docs (http://docs.spring.io/spring-data/rest/docs/1.1.x/reference/html/intro-chapter.html) show that the output should be:

{
    "links" : [ {
        "rel" : "customer",
        "href" : "http://localhost:8080/customer"
      }, {
         "rel" : "profile",
         "href" : "http://localhost:8080/profile"
      }
 }

Does anyone know why this is?

=====================================

Edit 14/08/14: I have taken my debugging a step further. By providing my own implementation of a org.springframework.hateoas.ResourceSupport class, which inspects the json for "_links" rather than "links" I get a step further. The error is:

"Can not deserialize instance of java.util.ArrayList out of START_OBJECT token ..... through reference chain: com.ebs.solas.admin.test.SolicitorDTO[\"_links\"]"

This is because the org.springframework.hateoas.ResourceSupport class seems to require that the links attribute be a json array. And by default the json+hal output produced by Spring Data for a Rest Entity does not produce an array (there are no square brackets):

"_links" : {
  "self" : {
    "href" : "http://localhost:9090/solas-admin-data-api/solicitorFirms/Fxxx"
  },
  "solicitors" : {
    "href" : "http://localhost:9090/solas-admin-data-api/solicitorFirms/Fxxx/solicitor
  }
}

Hopefully someone from the Spring forums could help me here.

==============================================

please see an outline of my Spring Data repository code:

@RepositoryRestResource
    public interface SolicitorFirmRepository extends CrudRepository<SolicitorFirm, String> {
}

@Entity
@RestResource
@Table(name="XXXX", schema = "XXX")
public class SolicitorFirm implements Serializable {
}

This successfully generates the following hateoas resource:

{
"firmNumber" : "FXXXX",
"solicitorType" : "r",
"companyName" : "XXXX",
"address1" : "XXXX",
"address2" : "XXX",
"address3" : "XXX",
"address4" : null,
"phoneNumber" : "XXXXX",
"faxNumber" : "XXXXX",
"county" : "OY",
"_links" : {
    "self" : {
        "href" : "http://localhost:9090/solas-admin-data-api/solicitorFirms/XXXX"
    },
    "solicitors" : {
        "href" : "http://localhost:9090/solas-admin-data-api/solicitorFirms/XXXX/solicitors"
    }
 }

HOWEVER, when i define a DTO for clientside/controller use:

import org.springframework.hateoas.ResourceSupport;
public class SolicitorFirmDTO extends ResourceSupport {
   .....
}

and use the following code

RestTemplate rt = new RestTemplate();
String uri = new String("//xxxxx:9090/solas-admin-data-api/solicitors/Sxxxxx");
SolicitorFirmDTO u = rt.getForObject(uri, SolicitorFirmDTO.class, "SXXXX");

I get the following error:

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "_links" (class com.ebs.solas.admin.test.SolicitorFirmDTO), not marked as ignorable (7 known properties: xx])

For some reason the json produced by Spring Data Rest adds the entity links under _links while the HATEOAS resource superclass expects links?

Can any one help? is this a version issue or do I need some extra configuration to map _links to links

I have tried MappingJackson2HttpMessageConverter and various media types application/json+hal to no avail.

Solution 1

Also with mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false) use @JsonIgnoreProperties(ignoreUnknown = true) on every Entity:

@Entity
@JsonIgnoreProperties(ignoreUnknown = true)
public class User {
    ...

}

Solution 2

For Spring-boot 1.3.3 the method exchange() for List is working

public void test1() {

    RestTemplate restTemplate = restTemplate();

    ParameterizedTypeReference<PagedResources<User>> responseTypeRef = new ParameterizedTypeReference<PagedResources<User>>() {
    };

    String API_URL = "http://localhost:8080/api/v1/user"
    ResponseEntity<PagedResources<User>> responseEntity = restTemplate.exchange(API_URL, HttpMethod.GET,
            (HttpEntity<User>) null, responseTypeRef);

    PagedResources<User> resources = responseEntity.getBody();
    Collection<User> users = resources.getContent();
    List<User> userList = new ArrayList<User>(users);

    System.out.println(userList);

}

private RestTemplate restTemplate() {

    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    mapper.registerModule(new Jackson2HalModule());

    MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
    converter.setSupportedMediaTypes(MediaType.parseMediaTypes("application/hal+json"));
    converter.setObjectMapper(mapper);

    List<HttpMessageConverter<?>> converterList = new ArrayList<HttpMessageConverter<?>>();
    converterList.add(converter);
    RestTemplate restTemplate = new RestTemplate(converterList);

    return restTemplate;
}

Comments

[Chris DaMour]

The output is HAL+JSON but your input is expecting just Spring's JSON data type. Do you need to talk HAL? Do you have @enablehypermediasupport annotation somewhere?

[Chris DaMour]

asking again..do you need to talk HAL? if you do..then your client parser needs to know how to deserialize it.

[APD]

Sorry I will be more specific. yes I do need HAL and obviously the client must be able to deserialize hal+json. If you look at the code I have posted then to the best of my knowledge I have configured/developed my client side code to do exactly that. It creates a Jackson2HalModule object mapper, a MappingJackson2HttpMessageConverter(with application/hal+json media type set) and configures both on the RestTemplate object. thanks

[Leonard Brünings]

You should edit your question instead of posting an answer which is just a reply to another answer/comment.

[APD]

sorry, my first question on stackoverflow. will do in future

[Florian Schaetz]

Thanks. The Converter for application/hal+json seems to do the trick just fine, otherwise RestTemplate isn't able to parse the whole "_link" thing.